clc;
syms n k positive;
assume(n,'integer');
a=2^n/symprod(2*k-1,k,1,n)%an
F=simplify(subs(a,n,n+1)/a),%an+1/an,这里的subs意思是把an中的n替换成n+1
L=simplify(limit(F,n,inf))
